The folks over at Campaign Monitor decided to run some tests to find out how well forms are supported in all the major email environments.
The short of it is that email clients consider email forms to be a security risk. While some email clients simply warn you of potential danger, others outright disable the forms. So if you want to send out a form, you should know that most of your recipients will never be able to use it. And for those who can, they’ll think twice about submitting data when they see a warning from their email client.
Campaign Monitor recommends linking to a form on a website in an email rather than embedding it therein. This is the safest, most reliable solution to pairing an email message with a form. More people will see it and be able to use it, and as a result participation will increase.
Source: Campaign Monitor